A heliostat typically includes one or more mirrors or other reflective surfaces the position and/or orientation of which are moved so as to keep reflecting sunlight toward a predetermined target despite the sun's apparent motions in the sky. Typically the target is stationary and the sun's light is reflected in a constant direction.
Some heliostats actively track and follow the sun, for example using light sensors. Others are controlled by a computer. The computer is given the latitude and longitude of the heliostat's position on the earth and the time and date. From these, using astronomical theory, the computer calculates the direction of the sun as seen from the mirror, e.g. its compass bearing (azimuth) and angle of elevation. Then, given the direction of the target, the computer calculates the mirror orientation required to reflect the sun's light to the target, and sends control signals to motors, such as stepper motors, so they turn the mirror to the correct alignment. This sequence is repeated to keep the mirror properly oriented.
Heliostats have been used in solar power applications, to direct the sun's light continuously onto a photovoltaic or other solar cell. They also have been used in daylighting, i.e., using reflected sunlight to illuminate interior or even exterior spaces. For example, heliostats have been used to direct reflected sunlight into interior spaces, such as into a building through a window or skylight. In such uses, typically the heliostat has been positioned outside the window or skylight and used to direct reflected sunlight, for example at a perpendicular or other constant angle, into an interior space through a window, a skylight, or a solar tube or other indirect path. Such external heliostats are exposed to the elements and may require roof penetrations to mount them. In addition, external heliostats may not be practical or desired for use with preexisting skylights or windows.